1. Field of the Invention
The present invention relates to a method for manufacturing circuit structures integrated in a semiconductor substrate, in particular lines of a conductive material in sub-micron circuit structures.
Specifically, the invention relates to a method for manufacturing circuit structures integrated in a semiconductor substrate, comprising isolation regions, the method comprising the steps of:
depositing a conductive layer to be patterned onto said semiconductor substrate.
Particularly but not limited to, the invention relates to a method of forming lines of a conductive material on a semiconductor substrate, wherein the line spacing is closer than that allowed by conventional photolithography techniques and continuously variable. The following description makes reference to this application field for convenience of explanation only.
2. Description of the Related Art
As it is well known, transferring patterns that have been defined in a light-sensitive layer (mask) to one or more layers beneath is a basic technical step in the integrated circuit fabricating process.
Also known is that the microelectronics industry has long been pursuing a reduction in the size of the several circuit structures that comprise an integrated circuit.
A first prior solution to the problem of bringing the spacing of structures patterned on silicon down to a value below that allowed by conventional photolithography techniques is based on the controlled oxidation of a silicon oxide mask, used for patterning a layer beneath.
Although this first solution is advantageous under different points of view, it presents several shortcomings. This solution comes out to be particularly crucial if structures of a conductive material, such as conductive lines, are to be formed. This because the realization of the silicon oxide mask, while effectively achieving a high mask-to-conductive material selectivity, often harms the interfacing oxide layers as the mask is removed.
Alternately, the use of conductive or “hybrid” materials such as polysilicon, silicon nitride (Si3N4), silicon oxynitride (SiON), or silicide layers employed to realize the mask, guarantees the integrity of the beneath oxide layers, but it does not allow to optimize the mask-to-conductive material selectivity.
In other words, when plasma etching is used, as is conventional practice in order to pattern conductive material layers, the mask employed for layer patterning wears out too much during the etching process. On the other hand, a highly selective etch is does not assure vertical profile where the thickness of the materials exceeds limiting values.
A second prior solution to realize closely spaced lines of a conductive material provides for spacers to be formed along the lithographic mask sides for narrower gaps between the conductor material lines.
Although achieving this objective, not even this solution is devoid of shortcomings, mainly because such spacers on a photoresist mask introduce problems due to the material of the spacers that are incompatible with the light-sensitive resin.
Thus, neither of the above prior solutions are really suitable to pattern specific structures such as the floating gate regions of a flash memory, for example, where polysilicon lines need to be provided with vertical profiles on substrates in which isolation regions or thin interfacing oxide layers are realized.
The underlying technical problem of the present invention is to provide a method of forming lines of a conductive material in integrated circuits, which method has suitable structural features such to allow the lines of conductive material to be formed very closed to each other, without damaging any structures previously formed in the layers beneath, thereby to overcome the shortcomings of manufacturing methods according to the prior art.